1. Field of the Invention
This invention relates to a temperature-compensated crystal resonator unit, and more particularly to a small and inexpensive crystal resonator unit with a high temperature gain. In a crystal resonator unit according to the present invention, the temperature variation of a crystal resonator element thereof is kept small for variation of the ambient temperature of the unit, so that the frequency fluctuation of the unit for ambient temperature variation is minimized.
2. Description of the Prior Art
To suppress the frequency fluctuation of oscillation of a crystal resonator for variation of ambient temperature thereof, it has been known to form a temperature-compensated crystal resonator unit by combining a crystal resonator element with a ceramic heater with a positive temperature coefficient of electric resistance (to be referred to as a "ceramic member with a positive coefficient", hereinafter), which ceramic heater fulfills the function of self-temperature compensation. Different types of temperature-compensated crystal resonator unit have been known heretofore. In a typical conventional temperature-compensated crystal resonator unit, a crystal resonator plate element is housed in a vessel and a ceramic member with a positive coefficient is attached to the outer surface of the vessel in a heat-conductive manner, and this vessel is simply placed in a larger container. In another type of conventional temperature-compensated crystal resonator unit, a crystal resonator element is simply housed in a vessel having a ceramic member with a positive coefficient mounted thereon in a heat-conductive manner. It is also known to wind a regular metallic heater wire around a crystal resonator element and to attached a ceramic with a positive coefficient to the crystal resonator element in a heat-conductive manner, and to electrically connect the heater wire and the ceramic member in parallel.
In any of the above-mentioned conventional type crystal resonator units, when its ambient temperature thereof varies over a range of -30.degree. C. to +60.degree. C., the temperature of its crystal resonator element varies between 30.degree. C. and 40.degree. C. Thus, even after very careful adjustment, a temperature variation of about 10.degree. C. or more is inevitable at the crystal resonator element. Accordingly, the temperature gain of the conventional crystal resonator unit is usually 2 to 3, and even when very careful adjustment is made, the temperature gain is 7 to 8.5 at most. Here, the temperature gain is given by ##EQU1## Consequently, the oscillating frequency of the crystal resonator element fluctuates considerably with the ambient temperature variation.
It is also known to form a crystal resonator unit by combining a regular heater element with an electronic temperature control means, so as to reduce the fluctuation of the oscillating frequency of the resonator unit even when the ambient temperature varies. This type of the crystal resonator unit using the electronic temperature control means can provide a very high temperature gain, but it has shortcomings in that the unit becomes large and costly, so that its use has been limited to some special fields.
Accordingly, there has been a pressing need for a crystal resonator unit having a high temperature gain and yet being small in size and low in cost.